1. Field of the Invention
The present invention relates, in general, to the use of radiation for initiating or providing an effect and in particular to a manner and a polyethylene element which facilitates the concentration of a radiation intensity to a predetermined volume or at an interface or surface.
2. Description of the Related Art
Normally, if absorption is desired in a material or an element in order to generate heat or other changes, an absorber is added to facilitate the absorption. However, absorbers have a tendency, in addition to their main function of absorption, to change other physical or chemical parameters of the material in a non-desired manner. One such parameter may be the colour of the means or material, and another may be the mechanical strength, which may both be adversely affected by the presence of the absorber. Also, the normally used absorbers can be relatively costly.
In order to reduce these “side effects”, less absorber may be used. This may, however, reduce the overall absorption to a level where the desired effect is not seen.
The use of radiation for performing or triggering different processes may e.g. be seen in: GB 1528452, U.S. Pat. No. 5,702,771, U.S. Pat. No. 6,326,450, U.S. Pat. No. 6,492,019, U.S. Pat. No. 6,248,974, U.S. Pat. No. 6,229,114, WO 02/23962, EP 1331635, EP 0476865, EP 0126787, WO 00/20157, WO 03/007080, and DE 101 58 016 as well as in Russek U A et al: “laser beam welding of thermoplastics, Proc. SPIE—the international society for optical engineering: photon processing in microelectronics and photonics II: San jose, CA, USA, Jan. 27-30 2003, vol. 4977, 2003, pages 458-472, Bachmann F G: “laser welding of polymers using high-power diode lasers”, Proc of SPIE, vol. 4637, 2002, p: 505-518, and “laserstrahlschweissen von Thermoplasten in Durchstrahlverfahren” 1 Feb. 2000, BASF A G, AWETA THERMOPLASTE, Ludwigshafen, Del.
One particular application is laser welding, where radiation is transmitted to an interface between two abutting materials having different absorption of the radiation used. The radiation is transmitted through the material having the lower absorption of the radiation so that, due to its higher absorption of the radiation, the other material is heated at the interface to a degree where melting and welding occurs.
It has been found, however, that problems may occur in laser welding due to the relation between the absorption required to obtain the heating and the effects of the radiation in areas away from the interface. In fact, weakening or decolouration of the material may be the result of the welding due to radiation not absorbed close to the interface heating the material further into the highly absorbing material.
Thus, a number of situations exist where the absorption desired/obtainable is not sufficient to obtain the desired effect (such as heat, damping of the radiation or the like) at/in the interface/volume desired and especially for soft thermoplastic materials such as polyethylene (PE), low density polyethylene (LDPE), high density polyethylene (HDPE) and the like.
In a number of other applications, where a photo- or heat-induced process is desired, the same problem may occur. Radiation not absorbed at the desired positions may damage materials at other positions.